There has rarely been any trickery to Kenley Jansen's game, and it has rarely mattered. He throws his cutter basically all the time, in any and all counts. The hitters know it's coming, and when. It hasn't made a difference. It is, quite possibly, baseball's most unhittable pitch.
This year, Jansen got even more dominant, notably setting a Major League record by collecting 51 strikeouts before issuing his first walk. And so far this postseason, he has been even better than that, despite the higher level of opponent. He has whiffed 10 of the 22 batters faced, allowing only two singles. Twice, he has pitched more than an inning.
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How? This is a story about the Majors' best reliever using his most storied pitch less, and supplementing it with a complementary offering that looks like it's a lot more than just a "show-me" pitch. For years, Jansen has gotten by with his cutter. Now, as the Cubs are learning, hitters have to worry about his slider, too.
To start, let's explain that it's in no way hyperbole to have called Jansen a one-pitch pitcher. Nearly nine out of 10 pitches that Jansen has thrown since his big league debut in 2010 have been cut fastballs, making him one of only two pitchers (min. 100 innings) to throw his cutter more than 80 percent of the time in that span. The other? Mariano Rivera, the legendary pitcher he has long been compared to. In fact, Jansen's 88.2-percent cutter rate slightly topped the 87 percent from Rivera. Jansen has long relied on just one pitch because that's all he has ever needed.
And why not? Jansen's cutter has elite spin, 2,602 revolutions per minute (rpm) as compared to the Major League average of 2,333 rpm. It has elite velocity, 93.3 mph as compared to the Major League average of 88.4 mph. Throwing it 88 percent of the time, he has the Majors' third-highest strikeout rate since 2010 (40.1 percent). It was the cutter that earned him a five-year, $80 million contract in free agency last offseason, and if he'd done nothing else but throw that cutter indefinitely, he'd be on a Hall of Fame track.
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But this year was something else. Jansen's 1.32 ERA marked his full-season low, and the best of the 355 pitchers with 100 innings. His 42.3-percent strikeout rate was his best since 2011, and second best in the big leagues, while his 2.7-percent walk rate was third lowest. His first-pitch strike percentage reached a career-high 73 percent, a Major League best. When hitters did manage to put the ball in play, they didn't do it well; his 23-percent hard-hit rate was the eighth lowest of all 438 pitchers who allowed 100 balls in play. Looking at Expected wOBA, our most advanced Statcast™ metric that combines quality of contact with amount of contact, Jansen's .198 xwOBA was tops among any pitcher. 
Obviously, throwing more first-pitch strikes helped, but around midseason, Jansen also began throwing his cutter slightly less and his slider a bit more. (He also throws a little-used fastball around 3 percent of the time.) So far this postseason, it's up to 15 percent -- and nearly a full quarter of the time with two strikes.

This isn't all new, of course. Jansen has been toying with his slider on and off for years, and it has been at times a useful pitch. Last year, when he picked up the final five outs to close out Game 1 of the National League Division Series against the Nationals, he threw 26 straight cutters before getting Jayson Werth to swing through a slider.

Still, something has changed. Including posteason numbers, just look at the difference in Jansen's slider production from 2010-16 as compared to '17 to date.
Jansen, slider production 2010-16
.174 average against
41-percent swing-and-miss rate
Jansen, slider production 2017
.057 average against
53-percent swing-and-miss rate
It's so good that it actually stands out among other, more-used sliders. In 2017 (including the postseason), that .057 average against was fourth lowest of the 406 who had at least 25 sliders put in play. Going back to xwOBA (which measures both quality of contact via exit velocity and launch angle as well as strikeouts and walks), his .075 is first of those 406 pitchers. Remember, that's a stat that's entirely about the slider, though in admittedly small samples. Without his signature pitch -- though likely benefiting from the thought of it -- his slider is showing elite outcomes.
So far this postseason, Jansen has thrown 12 sliders, inducing 10 swings, three in-play outs, three swinging strikes and whiffs of Anthony Rizzo and Kristopher Bryant. As Bryant found out in Game 2 of the NLCS presented by Camping World, the slider is more than a change of pace pitch; after starting the Cubs' star off with two cutters, Jansen then dropped in two sliders. It was, according to research done by Baseball Prospectus, only the second time all season long that Jansen had used back-to-back sliders.

But what's behind the improvement? This might go back to something we first noticed in May, after the Dodgers swept the Cubs in Los Angeles, about how Dodger relievers were throwing high fastballs like no one else in the Majors.
"Their bullpen," Bryant said to MLB.com's Ken Gurnick on May 28, "every pitch was right there at the top of the strike zone, every single one to all of us. It was unbelievable."
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It was true then, and it's true now. The Dodgers' bullpen threw 12.5 percent of their fastballs at the top of the zone, well above the 9.6 percent of the second-place Red Sox and the 7.6 percent Major League average. Jansen's 30 percent was far above that, and placed him tied for 20th out of 293 pitchers with 200 fastballs. It has been a trend; in 2016, that was 26.8 percent, and in '15, it was just 20.7 percent.
As his cutters stay high, the sliders go low. When Jansen throws his slider, 84 percent of them arrive at the plate two feet or lower off the ground, well above the Major League average of 54 percent. In fact, the vertical break difference between his two best pitches is extreme, especially for two pitches generally considered to be very similar. Among all pitchers who threw at least 10 innings this year and threw both a cutter and a slider at least 2 percent of the time, no pitcher had a larger difference in vertical break between their cutter and slider than Jansen. (Interestingly, if we'd gone with fewer than 10 innings, Jansen would have been second to his own teammate, rookie Walker Buehler.)
The cutter is always going to be Jansen's primary pitch, of course. But the slider isn't just a nice change of pace, it's an actual out pitch. Imagine that, won't you? As though trying to handle cutters up weren't hard enough, now there are sliders down. Jansen was already in the conversation for the Majors' most dominant reliever, and now he's getting even better, just like Kenta Maeda using his cutter more, just like Tony Cingrani using his slider more. It hardly seems fair.